This invention relates to a wide band magnetic recording system, and particularly to types of scanners and scanning formats for the magnetic recorders in which the transducer heads are continuously scanned at a relatively higher velocity past a moving record media, such as recording tape, which is moving at a substanilly lesser velocity.
A number of different types of scanners have been known. One of the first in wide commercial use was the so-called transverse scan, quad format, which is typified by the many presently used studio television recorders. It is disclosed in U.S. Pat. Nos. 2,912,518, and 2,956,114. A relatively wide tape is moved along its length, and at the recording station the cross section of the tape is formed to take the shape of a section of a cylinder. The arc of the section may be slightly in excess of 90 degrees.
The transducer heads are mounted on a wheel which has an axis of rotation parallel to the longitudinal center line of the tape path, and the transducer heads rotate in a common circle or orbit, consecutively scanning transversely of the recording tape. The tape motion spaces apart the successive scans of the heads and the "wrap" of the tape around the transducer orbit is such that one head begins recording interface with the tape before the preceding head leaves the tape. Each head scans essentially the full width of the tape, and the angle of the scans with respect to the length of the tape is a relatively large angle, usually in excess of 60 degrees and closer to 90 degrees. This angle, of course, will vary with differences in velocities of tape movement and head rotation.
Another form of scanner is commonly referred to as the helical scan, and it in turn is embodied in several different ways. In most cases the scans are at a much lesser angle to the tape length, such as between 30.degree. and 60.degree.. For example, a single head is rotated in a circle or orbit, and the recording tape is wrapped around the mandrel or drum on which the head rotates, either with the drum itself rotating or the head rotating in a slot around the drum surface. The tape is wrapped completely around the drum (360.degree. wrap) and the single head thus produces successive scans at an angle across the tape. This angle will depend again on the relative head and tape velocities, and also on the diameter of the head orbit and the width of the tape, each of these being a factor in determining the helix angle at which the tape wraps acound the drum.
Another version of the helical scan recorder utilizes the so-called omega wrap, wherein the tape extends around the scanner drum or mandrel in a loop which is slightly open, a few degrees less than 360.degree.. Again, it is most common in this type of system to utilize a single head, and the scanner diameter, tape width, and the head and tape velocities are selected and correlated such that when used for television recording, this system normally scans at a rate such that one scan of the head at an angle across the tape corresponds in time to one field of video information.
Another version of scanner utilizes a tape wrap slightly in excess of 180.degree. and two diametrically opposed heads, as in U.S. Pat. No. 3,418,424. The heads transduce the same signal, and due to the wrap of slightly over 180.degree. there is a small amount of redundancy where one head leaves the tape and the other enters into interface with the tape. Scanners of this type have been proposed for single field recording, and in addition some such scanners have utilized a format where each scan occurs so rapidly that only a segment of any video field is recorded. In such instance the scanner system has been known in the art as the "segmented field" recorder and scanner.
Various other scanner arrangements have been proposed utilizing three transducer heads instead of two, with a 120.degree. wrap, or four heads with a wrap slightly in excess of 90.degree.. Such scanners have been found, for example, in systems which were introduced to the commercial market, but have since been discontinued for one reason or another.
As mentioned previously, some of these scanners utilize a rotating drum or wheel (the terms are often used syonymously, but larger diameter rotating members have also been called wheels), to carry the heads, while in units with smaller diameters the heads rotate in slots formed in stationary drums or mandrels. With a few exceptions, however, when a plurality of heads is used they have been arranged to follow the same orbit or circle.
One exception to the common orbit scanner is a single field omega wrap scanner, disclosed in U.S. Pat. No. 3,188,385, where two heads are utilized spaced axially of the scanner as well as circumferentially. The circumferential spacing is non-uniform, one head lagging the other by an acute angle in the order of 8.degree. to 10.degree.. In video recording, one head receives the video information for an entire field, and the other head receives only vertical synchronizing information which is recorded in a separate path or track along one edge of the tape. These vertical sync scans are substantially lesser in width than the main scans on which full field information is recorded.
Another exception is the skip field recorder such as disclosed in U.S. Pat. No. 3,588,378. There a single head is used to scan helically and record one of every three fields of a television signal. The scan extends the full width of the tape. Three playback heads are used, spaced 120.degree. apart. The head to tape relation is mechanically phase shifted from record position to playback position, so that each of the three playback heads tracks in sequence the same single scan previously followed by the record head. For this purpose, the playback heads are axially displaced a small amount, but the scanning helix angle is such that each of the heads scans fully across the tape.
All of the above mentioned scanners, except the transverse scan quad format, require either a substantial change in elevation of the tape in passing around the scanner drum, or use of a rather large drum mounted with its axis of rotation at a small angle to the tape path. The transverse scan, of course, requires a semicylindrical forming of a longitudinal section of a wide tape, bending it generally about its longitudinal axis, and this presents increasing difficulties as narrower tape is used. As a rule, four inch wide tape is used with the transverse scan quad format.
When change in tape elevation is encountered with narrower tapes (e.g., two inch or less) helical scan formats which use a smaller diameter and greater angle of scan, complex rollers, guides, etc., are required to control precisely the movement of the tape onto, around, and exiting from the scanner.
Various proposals have been made for scanners which cooperate with a cartridge or cassette (the terms are sometimes used interchangeably) which provides a convenient mechanism for storing and handling the recording tape. A number of systems have become known commercially, as follows.
The first system involves a two-head scanning wheel with mechanism that reaches into a cassette and withdraws the tape, forming the tape into a loop of more than 180.degree. around the scanner. Such a system is utilized commercially by a number of companies such as Sony, Phillips and JVC.
A second arrangement, marketed by a company known as Cartridge Television, Inc., utilizes a scanning wheel with three heads. The tape is again, however, withdrawn from the cassette and formed into a turn of slightly more than 120.degree. around the scanning wheel.
A third system, which has been proposed by companies such as Ampex and Matushita, involves the use of a cartridge with a single spool, having a free end of tape extending from the cartridge with a leader thereon, together with a self-threading mechanism that directs the leader and tape around a two head scanning wheel, wrapped in excess of 180.degree., and thence to a take-up spool which is separate from the cartridge.
A fourth type of cartridge system was introduced by RCA, and utilizes a cartridge with an opening that can be uncovered in one of its edges to expose a span of tape, the space behind such span being somewhat open within the cartridge, and a four head scanning wheel. This is shown in U.S. Pat. No. 3,766,328. The cartridge is inserted around the circular periphery of the scanning wheel, such that the span of tape forms a turn within the cartridge contacting the scanning wheel over an arc slightly in excess of 90.degree..
All of the aforementioned cartridge (or cassette) systems, however, utilize rather large scanning wheels, having diameters in the order of six to nine inches, such that the tape wrapped around the scanning wheel is of considerable extent.
Another problem of rotary scanners is concerned with head tracking at different tape speeds. Since the rotational speed of a scanner is considerably greater than the longitudinal tape speed, the ultimate head-to-tape speed is only slightly affected by changes in this longitudinal speed. Therefore, it is possible to slow the tape and still recover recorded information at a satisfactory signal/noise ratio. However, this slowing of the tape does introduce tracking errors due to a change in the angle at which the scanning head moves across the tape. In prior art rotary head recorders, since a head scan proceeds entirely across the tape, the tracking error is significant, and various devices, such as servo-positioning of the head during playback, have been proposed to improve tracking.